Neck Tension Conundrum

I have been thinking a lot about neck tension and what most feel it should be. If the neck tension is consistent on all shells loaded, what does it matter if the neck diameter is 2-3-8 or 10 thou under bullet diameter.

I have been thinking a lot about neck tension and what most feel it should be. If the neck tension is consistent on all shells loaded, what does it matter if the neck diameter is 2-3-8 or 10 thou under bullet diameter.

It matters because if you are more than a couple of thou. under bullet diameter the brass will tend to stretch offset a bit and you will be introducing run-out into your ammo.

IMO trying to press a bullet into a case with a difference much past .004" is asking for problems. The neck could be pushed out of alignment causing excessive bullet runout translating to poor accuracy.

You didn't factor neck wall thickness into your theory. Using thin (.010") or thick (.014") neck walls can change neck tension complicating the issue on how much to generate with your dies.

Best to stick with what has shown to work. If you can go with .001" or .002" difference between sized and seated diameters you will get best accuracy for single shot application. You may need more tension if the rifle is a repeater especially if it has high recoil. A crimp may be needed in some cases, the 375 H and H comes to mind. Typically RCBS full length dies create a .003" neck tension difference.

When the spread in neck tension (called release force in the ammo industry) increases, so does muzzle velocity. It's typically about 20% about the average amount. Lighter is always better than heavier for accuracy purposes. But too little won't hold the bullet in place during normal handling.

A .001" smaller case mouth diameter (after sizing) than bullet diameter is about right for most uses for bullets up to 200 grains.

Interesting. I found my RCBS full length die was around .003 mark as well. I have bushings that stepped down to .002. My case neck loaded is .294 and the bushing is .292 that I use. Fired empty is .296.

Having said all that I tried a smaller bushing for more tension and the rifle didn't seem to care for it that much and I noticed a bit more kick and cratering in the primer. Guess that is what these are refering too, more pressure to get the bullet moving.

I've done some serious experimenting with neck tension in 22 and 6mm calibers, and one thing I learned fast was that there will be a sweet spot in the amount of tension that will greatly make a difference is your extreme spread in velocity. One thousandth of an inch took my ES from double digits (close to twenty fps) down to about seven or eight fps on a .223 case. The 6mm Remington also seems to be touchy on this issue alone, but not as much as the smaller case was. Now I starting to experiment with my 6/250AI.
gary